Immunoassay screen for qualitative detection of barbiturates in urine.

Drug abusers may abuse various barbiturates, such as short-acting secobarbital and long-acting phenobarbital, through oral ingestion or by intravenous and/or intramuscular injection. Long-term abuse can lead to respiratory depression or, in severe cases, coma. When ingested, a barbiturate is rapidly metabolized and excreted into urine, allowing immunoassays to detect recent use. The barbiturate screening assay uses monoclonal antibodies that detect most barbiturates in urine. The assay is based on the competition of an enzyme glucose-6-phosphate dehydrogenase (G6PDH) labeled drug and the drug from the urine sample for a fixed amount of specific antibody binding sites. In the absence of the drug from the sample, the G6PDH labeled drug is bound by the specific antibody and the enzyme activity is inhibited. This phenomenon creates a relationship between drug concentration in urine and the enzyme activity. The enzyme G6PDH activity is determined spectrophotometrically at 340 nm.